U.S. Pat. No. 3,816,840 describes an electrostatic printing process and printer in which a dielectric image-receiving material is fed between a first and a second electrode which are disposed a short distance apart. One of the electrodes is covered with a layer of magnetically attractable electrically conductive toner powder. Voltage pulses are applied between the electrodes so that tone powder is deposited on the image-receiving material in the form of an information pattern. A disadvantage of this process is that only dielectric image-receiving material can be used, thereby restricting the choice of usable image-receiving materials.
U.S. Pat. No. 3,946,402 describes an electrostatic printer comprising a rotatable drum provided with a dielectric layer on which a uniform layer of electrically conductive magnetically attractable toner powder is applied. A magnetic roller is disposed in an image-forming zone near the drum surface covered with toner powder and has a stationary nonmagnetic sleeve and a rotatable magnet system mounted inside the sleeve. A large number of magnetic electrodes in the form of rods each connected to a voltage supply are disposed axially on the sleeve of this magnetic roller. When the electrodes are not energized, toner powder is attracted from the drum surface to the magnetic roller. No toner powder is attracted when the electrodes are energized. By energizing the electrodes pulse-wise according to an information pattern, a toner image corresponding to the information pattern is formed on the drum and can then be transferred to a receiving support.
Since the electrodes are conductive they must be insulated from one another. A disadvantage of this device is that the conductive toner powder can short-circuit some electrodes and thus disturb the image formation. Another disadvantage is that it is a very complex and expensive matter to construct the row of fine magnetic electrodes in rod form used in this device.
Japanese Application No. 59-224369 shows an image-forming element comprising an endless support with an insulating layer and a multiplicity of electrodes embedded in the insulating layer. Preferably, the electrodes form rings around the circumference of the support and are selectively electrified from within the support. One embodiment, however, shows the electrodes extending in the shape of a helix. No provision, however, is made for electrifying the electrodes on the surface of the support or doing so outside the image-forming zone. Indeed, electrifying the electrodes from within the suport, as this device teaches, is not easy to do and requires the construction of a special support. A better way of electrifying the electrodes is needed.
Spiral electrodes have been used before as can be seen from Japanese Application Nos. 58-72474 and 58-171975 and German Application No. 28 37 828. The spiral electrode in Japanese Application No. 58-171975 is provided on the surface of the cylindrical support. No insulating layer covers it. Instead, the toner used must be insulating. The same appears to be true with German Application No. 28 37 828. In Japanese Application No. 58-72474, while there is an insulating layer, this layer is peeled off in a straight line to expose the conductors and form the multistylus electrode. If conductive toner is used with any of these structures, it can short-circuit some electrodes and thus disturb the image formation.
Accordingly there is a need for an image-forming element for an electrostatic printer which obviates the above-mentioned disadvantages.